The enzyme rhodanese (thiosulfate sulfurtransferase; TST; EC 2.8.1.1) offers an unusual opportunity to study and understand features of protein structure and structural fluctuations that are important in enzyme catalysis and protein architecture. The enzyme rhodanese itself is of interest as the function of this ubiquitous protein is becoming better defined. For the present work rhodanese represents a system which can be studied in a detail which has rarely been achieved. The kinetics of the enzyme are well understood; the x-ray structure is known to 2.5 A resolution for one enzyme form; it displays conformational changes during catalysis; it is folded into distinct structural domains with the active site in the interdomain region; and it displays a number of non-artifactual discrepancies between the x-ray and solution studies which reflect the flexibility of this protein. At the same time the enzyme has the appropriate properties to permit study by a complete array of fluorescence and corroborative techniques. The proposed research is designed to continue a program which seeks to establish and pursue correlations between the possibly flexible conformational states of rhodanese and the catalytic events in which it participates. Steady-state fluorescence and fluorescence life-time techniques will be used as the major methodology and the results will be corroborated with data from x-ray analysis, other physico-chemical techniques and kinetic studies. It will be of particular interest to pursue recent findings from this laboratory that there are low energy structural transitions in rhodanese and other proteins which are folded into functionally relevant domains. An underlying objective of this research is to develop a set of diagnostics, approaches and techniques for the recognition and study of the role of protein flexibility and domain interactions in other functional systems.